Contamination with minute particles must be strictly avoided in precision substrates such as silicon wafers, glass plates for photomasks and the like. It is an essential requirement in transportation or stock-keeping of these precision substrates that the work therefor is undertaken in an extremely clean environment so that the substrate materials are handled by an automatized machine in order to avoid dust occurrence and contamination from the workers' bodies and clothes.
Even as contained in a precision substrate-container vessel (referred to as a container vessel hereinbelow), it is possible that precision substrates will become contaminated by rubbing due to contact with the inner walls of the container vessel and by a trace of gaseous emission from the resin forming the container vessel.
It is therefore essential to the selection of the material of the container vessel to select a material which ensures the least amount of contamination of the precision substrates. In particular, selection of the material of the presser members coming into direct contact with the precision substrates is an important key requisite.
As a material constituting the presser members, for example, a proposal has been made to use a material as described in the official publication of Japanese Patent Kokai No. 11-163115. The material which is subjected to a treatment for reducing generation and emission of contaminant gases.
Since contamination with minute particles must be strictly avoided in precision substrates, as is mentioned before, it is desired that the outer environment for using and stock-keeping of the container vessel and the environment inside the container vessel are kept as clean as possible.
Along with the recent trend in the semiconductor industries toward larger and larger diameters of the precision substrates, higher and higher densities, and higher and higher degrees of integration in semiconductor devices, the inner and outer environments of the container vessel are also required to be kept in a condition of still higher cleanness.
In compliance with the trend in precision substrates toward larger sizes or larger diameters, on the other hand, container vessels of a large capacity have now come under use with a consequent increase in the surface area and thickness of the presser members, resulting in an increase in the volume of emitted gases responsible for contamination of the precision substrates so that it is now necessary to decrease the absolute amount thereof.
Generally stating, thermoplastic elastomer materials forming the presser members contain low molecular weight constituents and additives in a relatively high content, as compared with resinous materials. As a result, greater volumes of gaseous emission are generated as a matter of consequence.
By virtue of the progress of analytical methods in recent years, it has now become possible to achieve a good accuracy in conducting analysis of the kind and amount of emitted gases from a molded article by a method (the so-called head-space method) for qualitative as well as quantitative analyses analyses. In the method, the objective material as sealed in a hermetically sealable vessel is heated at a specified temperature of, for example, at 60° C. for a specified length of time of, for example, 1 hour followed by collection of the gas in the hermetically sealable vessel to be subjected to gas chromatography, mass spectrometry and other processes.
As a result, it has now become clear that the amount of gaseous emission from a polyolefin-based or polyester-based thermoplastic elastomer is greater by about 10 times to 50 times than that from a thermoplastic resin such as polypropylenes, polycarbonate, polybutylene terephthalates and the like as a conventional material for forming container vessels. In addition, thermal decomposition of these materials causes generation of a volatile matter such as tetrahydrofuran which, once deposited on a precision substrate, cannot be removed completely without difficulties.
Accordingly, it has now become an important problem to take a countermeasure against gaseous emission from a presser member formed from a thermoplastic elastomeric material which is liable to emit a large amount of gases so as to be a contamination source.
Besides, a presser member formed from a thermoplastic elastomer alone has a defect in that the amount of creeping deformation is so large that the holding power is decreased in the course of long-term storage and cannot be re-used. As a result, replacement with a fresh member is necessary because changes are caused in the pressing position and holding power on the precision substrates between a member even after a single use and a fresh member resulting in the lack of reproducibility in the holding conditions of the substrates.
The present invention, which has been completed in consideration of the above-described problems, provides, in containing precision substrates in a container vessel, a container vessel capable of containing precision substrates smoothly and in an optimum press-holding condition to be suitable for reducing the amount of gaseous emission from the presser members, which exhibits adverse influences when precision substrates are contained in the container vessel as well as a presser member therefor.